Major General Gordon Messenger said there was "insufficient" manpower in Helmand 18 months ago and that his own brigade had been "stretched" and unable to venture into certain areas.

US President Barack Obama's deployment of an extra 30,000 troops came from "the realisation that the scale of the challenge was not matched by the resources allocated to it", he told the Commons Defence Select Committee.

MPs on the committee questioned why it had taken so long into the nine-year conflict for the Nato coalition to work out that more troops were needed. Ministers have repeatedly insisted that all requests from military commanders in Afghanistan have been met.

Gen Messenger, strategic communications officer to the Chief of the Defence Staff, said only now had adequate resources been in place for long enough to be making a positive impact.

"In places like Helmand, which is as bad as it gets in terms of security across Afghanistan, we've started seeing far more positive indicators," he said.

"I commanded a brigade, alongside an Afghan brigade commander, that were stretched and were not able to go to certain key areas where we knew we would ultimately have to go in order to secure the population.

"What has happened since has been an enormous inflow, of principally Americans but also other Nato nations, and a huge upsurge in the number of Afghan National Army and Afghan National Police that are in the line providing security.

"The picture now is a situation where the key areas of population in Helmand are now secured.

Committee chairman James Arbuthnot, a Conservative MP, told Gen Messenger: "You are an immensely reassuring man... but it would be more reassuring if you told us 18 months ago that we were getting things wrong, but I can't remember your doing so."